[Build] Restore operational AsyncDisplayKit-Prefix.pch

This was regressed by the large project file changes in 404795dc02

I don't yet have full context on the goals of that change, but it's definitely necessary to have the .pch
with the current requirements of the project. This line ensures that Cocoapods adds it.

AsyncDisplayKit.podspec

@@ -19,6 +19,7 @@ Pod::Spec.new do |spec|
 
   # Subspecs
   spec.subspec 'Core' do |core|
+    core.prefix_header_file = 'AsyncDisplayKit/AsyncDisplayKit-Prefix.pch'
     core.public_header_files = [
         'AsyncDisplayKit/*.h',
         'AsyncDisplayKit/Details/**/*.h',